Conventional engine liquid cooling systems generally use water-based coolants. A commonly used water-based coolant is about 50% water and 50% ethylene glycol (by weight) with additives to protect against corrosion. Such coolants are typically referred to as "antifreeze."
A water-based coolant system is pressurized during vehicle operation by the thermal expansion of the coolant and by the water vapor generated upon localized coolant boiling. The engine radiator is typically equipped with a pressure relief valve that limits the system pressure to about one atmosphere above ambient pressure. An overflow reservoir is provided to hold the coolant purged from the radiator when the pressure relief setting of the valve is exceeded. A second valve is provided to permit the coolant to return to the radiator when the pressure within the radiator falls below the ambient pressure.
Although the water-based ethylene glycol coolants exhibit low freezing points in comparison to water, their boiling and condensation characteristics are similar to that of water. The saturation temperature of water, which is its boiling point and maximum condensation temperature, is about 100.degree. C. at 0 psig and 115.degree. C. at 15 psig; whereas the boiling point of a 50/50 water/ethylene glycol coolant is about 107.degree. C. at 0 psig and 124.degree. C. at 15 psig. Water, however, exhibits a substantial vapor pressure in comparison to ethylene glycol. Therefore, when a 50/50 water/ethylene glycol mixture is boiled, the vapor generated is about 98% water (by volume). At one atmosphere pressure (gauge), the water vapor does not condense above 121.degree. C.
Under heavy load and/or high ambient temperature conditions, the coolant temperature frequently approaches the saturation temperature of water. As a result, the water vapor cannot condense quickly enough to prevent it from occupying and insulating critical areas within the cylinder head. Hot spots develop where the liquid coolant is displaced by vapor from the hot metal surfaces of the engine. Hot spots can cause detonation and excessive NOX emissions.
One approach to preventing detonation is to remove the spark advance. Another approach, used particularly with engines having electronically controlled fuel injection, is to enrich the air to fuel mixture. With turbocharged engines, the turbo air pressure, or boost, can be reduced when the coolant temperatures approach the saturation temperature of water. The problem with these approaches is that each causes a loss of engine performance and/or a decrease in fuel economy.
The ability to control hot spots and detonation is directly related to the ability to condense vapor in the cylinder head. In liquid cooling systems, the temperature of the coolant in low pressure regions, such as upstream of the coolant pump, must be maintained sufficiently below the boiling point of the coolant to prevent flash vaporization. Flash vaporization of the coolant immediately upstream of the pump can cause pump cavitation and, as a result, a sharp decrease in coolant flow. Cavitation is most likely to occur at high pump speeds and/or under high pump suction forces, when the pump input pressure is lowest. Once the coolant flow is interrupted, the coolant can quickly increase in temperature and lead to a total failure of the cooling system.
Conventional cooling systems try to prevent cavitation by drawing lower temperature coolant from the radiator rather than the higher temperature coolant from the engine coolant jacket. The coolant flows from the outlet of the pump, into the engine block, and up through the cylinder head. The coolant entering the cylinder head is therefore preheated by circulation through the lower part of the engine. One problem, however, in pumping the coolant in this direction is that the higher temperature coolant entering the cylinder head is less likely to control the formation of hot spots and detonation.
For water-based coolants, the failure point of the system is the saturation temperature of water, regardless of the concentration of other constituents, such as ethylene glycol. For example, a coolant mixture which is 90% ethylene glycol and 10% water (by weight) will still yield vapor that is about 90% water (by volume) when boiled.
Therefore, with water-based coolants, it is critical that the bulk coolant temperature in the cylinder head not exceed the saturation temperature of water under all operating conditions. The bulk coolant temperature must be maintained below that level if the bulk coolant is to condense the water vapor generated upon contact by the coolant with the hotter metal surfaces of the engine. When that temperature limit is exceeded, none of the water vapor generated can condense. As a result, a large volume of vapor is generated that forces substantial amounts of coolant into the overflow reservoir. The engine must then be stopped immediately to prevent severe damage from the coolant loss.
Certain problems arise, however, in maintaining the temperature of water-based coolants below the saturation temperature of water. Because the lower temperature coolant is pumped into the engine block, and then up through the cylinder head, the cylinder walls are frequently maintained at relatively low temperatures. The low temperature cylinder walls can prematurely quench the combustion flame. As a result, a boundary layer of unburned fuel can develop on the inner surfaces of the cylinder walls. Although the unburned fuel might oxidize before it is exhausted, it is not converted into usable mechanical energy.
Another problem with water-based coolant systems is that vehicle designs employing down-sized radiators, or that reduce the air flow through the radiator, are difficult to implement. Water-based coolant systems usually only maintain a slight difference between the bulk coolant temperature and the saturation temperature of water under heavy operating loads and/or high ambient temperatures. Therefore, the radiators in water-based coolant systems are required to maintain a relatively high rate of heat exchange with the coolant. The required heat exchange rates frequently cannot be maintained with a down-sized radiator, or if the flow rate of air through the radiator is reduced.
Another drawback of water-based coolant systems is that there are substantial benefits in maintaining controlled coolant temperatures well above 100.degree. C.--an operating regime not ordinarily achievable with water-based coolants. By operating with higher temperatures in the cylinder bores, there is less heat rejected from the engine and thus greater engine efficiency. Carbon monoxide (CO) and hydrocarbon (HC) emissions are reduced because there is a more complete burning of the fuel. Conventional water-based coolant systems can only attempt to operate at such high temperatures by increasing the pressure of the system. A high pressure coolant system can be very dangerous, however, particularly because many common coolant constituents, such as ethylene glycol, are toxic and flammable. Moreover, the high pressure conditions typically decrease the life of a coolant system's components, such as hoses, clamps, the pump, and the radiator.
There have been attempts to develop engine cooling systems that do not use water-based coolants. However, each of the known attempts have certain drawbacks or disadvantages that have prevented them from attaining widespread acceptance.
U.S. Pat. No. 4,550,694, dated Nov. 5, 1985, to the same inventor as the present application, shows an apparatus for cooling an internal combustion engine using a boilable liquid coolant having a saturation temperature above 132.degree. C. The vapor generated rises by convection to the highest region or regions of the head coolant jacket. The vapor is then removed through several outlets and conducted through a conduit to a vapor condenser.
The condenser is located above the head coolant jacket in all orientations of the engine in normal use so that the condensate from the condenser can be returned to the engine by gravity through either a return conduit or the same conduit by which the vapor is conducted into the condenser. The condenser is an elongated vessel mounted under the vehicle's hood lengthwise of the engine compartment, sloping up from front to back.
A vent pipe leads from a region high in the condenser and remote from the vapor inlet. A two-way pressure relief valve in the vent pipe blocks the passage of gases from the condenser through the vent pipe until the pressure increases to a predetermined level. When the valve opens, gases from the top of the condenser flow into a recovery condenser, a small vessel located in a place likely to be cool at all times. By choosing a relatively high setting for the valve, generally on the order of 70 kPa (10 psi), the cooling system is effectively closed except under unusually heavy load conditions or large changes in altitude.
The apparatus of the '694 patent can use substantially anhydrous coolants and, therefore, derive certain benefits over water-based coolant systems therefrom. However, one disadvantage of the apparatus is that it requires a condenser. The condenser is relatively bulky and must be mounted above the engine so that it is located above the highest coolant level. This limited flexibility prevents the use of the apparatus in many types of vehicles. And those vehicles that can use the apparatus are limited to only certain designs that can accommodate the condenser. Moreover, the condenser can add a significant cost to producing the cooling system. Its advantages in performance, therefore, frequently do not outweigh its disadvantages with regard to cost and design flexibility.
It is an object of the present invention, therefore, to overcome the problems of known engine liquid cooling systems.